1. Field of the Invention
This invention relates to switch assemblies and more particularly, but not exclusively, to rope operated switch assemblies used to control the power supply to kinetic machinery.
Known rope operated switch assemblies comprise a safety switch adapted to be fitted in proximity to a machine. and an actuator connected to the switch and operable by a rope to turn off the electrical power supply when the rope is pulled or slackened.
2. Discussion of the Related Art
Safety switches of this type have a housing in which are situated normally-open contacts, one set fixed, the other movable and carried by an axially-movable plunger spring-loaded to maintain the sets of contacts closed and the power supply consequently on.
The axially-movable plunger bears against a rotatable cam of a cam arrangement normally disposed to maintain the cam in a position such that the plunger is in a power supply ON position but which is operable by the rope to cause cam rotation and axial movement of the plunger to a power supply OFF position.
In one particular example the rope is connected in axial alignment to a shaft extending into the housing. The rope is connected to the shaft in tension so that the shaft is held against a biasing force provided by a shaft spring. If the rope tension is relaxed (e.g. by cutting it) the biasing force moves the shaft in a first axial direction and if the rope tension is increased (e.g. by pulling it) the shaft moves in a second axial direction with the rope. The shaft has a circumferential latch adjacent an undercut at a certain position along its length. The latch is biased in a direction perpendicular to the longitudinal axis of the shaft. The shaft carries a loop that engages a pin on the cam so that axial movement of the shaft in either direction will drive the cam and operate the plunger to a power supply OFF position. If the rope is cut, the shaft spring maintains the shaft in a switch OFF position. If the rope is pulled but subsequently released, the shaft is maintained in a switch OFF position by the latch which has engaged with the undercut. This prevents the power supply being turned on again unless the switch is reset.
These known switch assemblies operate satisfactorily if the rope is cut, but suffer from the disadvantage that if the rope tension is increased slightly it may be sufficient to turn the power supply off but not sufficient for the latch to operate so as to prevent a subsequent slight reduction in rope tension turning the power on again. Similarly, if the rope tension is decreased slightly it may be sufficient to turn the power supply off but a subsequent slight increase in tension could turn the power supply on again. Thus dangerous conditions can arise if for example a machine operator has been injured and has pulled the rope to switch off the machinery but it too weak or is otherwise unable to pull on the rope with sufficient force to engage the latch.
It is an object of the present invention to provide a switch assembly in which such disadvantages are obviated or mitigated.
According to the present invention there is provided a switch mechanism comprising a switch which is actuable to switch between first and second conditions, a cam follower movement of which actuates the switch, a rotatably mounted cam against a surface of which the cam follower bears, an actuator member which is displaceable relative to the cam, and a linkage between the actuator member and cam arranged such that displacement of the actuator member from a predetermined position causes the cam to rotate and actuate the switch, wherein the linkage comprises a lever mounted on a pivot that is displaced with the actuator member, the lever co-operating with a surface of a stationary cam such that displacement of the actuator member causes the lever to pivot and rotate the rotatable cam.
The lever arrangement in accordance with the present invention makes it possible for a relatively small movement of an actuator shaft or the like to cause a relatively large angular movement of the rotatable cam. Thus the device is particularly sensitive which is of real importance in the case of a rope-operated switch.
The actuator member may be a shaft that extends into a housing, the lever being pivotally supported on a body connected to the shaft and the stationary cam being defined by an internal wall of the housing.
Preferably the rotatable cam is rotatable from a datum position in which the switch assumes the first condition to at least one displaced position in which the switch assumes the second condition. and means are provided to bias the cam away from the datum position once the cam has been rotated by the lever. The biasing means preferably comprises a first latch member pivotally supported adjacent the rotatable cam, a second latch member bearing against the cam, and a spring arranged to bias the first and second latch members apart, the spring biasing force being directed in a direction which intersects the axis of rotation of the rotatable cam when the cam is in the datum position. The actuator member may extend through an aperture in at least one of the latch members, and the first latch member may be pivoted about an arcuate surface against which it is biased by the spring. Means may be provided to push the first latch member to a position in which the spring biasing force is directed in a direction to one side of the axis of rotation of the rotatable cam and thereby to cause the cam to rotate.
Preferably the lever defines a recess on one side which receives the stationary cam and a pair of arms on the opposite side to the recess which arms project on opposite sides of an abutment member forming part of the rotatable cam when the actuator member is in the predetermined position, one arm being displaced into contact with the abutment member when the actuator member is displaced in a first direction from the predetermined position, and the other arm being displaced into contact with the abutment member when the actuator member is displaced in the opposite direction to the first direction. The lever arms may be disposed such that rotation of the cam to actuate the switch between the first and second conditions is obstructed unless the actuator member is in the predetermined position.
In an alternative arrangement, the lever may define a recess on one side which receives the stationary cam and a single arm on the opposite side to the recess, the arm projecting to one side of an abutment member forming part of the rotatable cam when the actuator is in the predetermined position, and being displaced into contact with the abutment member so as to rotate the cam when the actuator member is displaced in a first direction from the predetermined position. And the actuator member supporting an abutment member which is displaced into contact with the biasing means so as to rotate the cam when the actuator member is displaced in a second direction from the predetermined position. Displacement of the actuator member in the first or the second direction may rotate the cam in the same direction.